The invention relates to the production of a graft copolymer of vinyl chloride (VC) onto a crosslinked butyl acrylate polymer, exhibiting improved processing properties, especially during injection molding, without there being a loss in good physical properties.
Numerous attempts have been made in the past to improve the processability of VC polymers, i.e., to reduce the viscosity of the polymer melt which, at a given temperature and shear rate, is dependent on the average molecular weight of polyvinyl chloride (PVC).
Thus, readily flowing polymer melts can be obtained by using polyvinyl chloride having a low average molecular weight with a narrow molecular weight distribution; however, these melts exhibit poor mechanical properties as finished articles [Pol. Eng. Sci. 17 (1977), 194 et seq.]. Therefore, methods have been developed with the objective of broadening the molecular weight distribution of polyvinyl chloride.
Thus, in accordance with DOS 2,651,532, a mechanical mixing of polyvinyl chloride powders having differing average molecular weights is performed. This process is, however, expensive on an industrial scale and yields powders processable on injection molding machines into smooth-surface finished parts only if the average molecular weights of the mixture components are not too different. The thus-attained lowering of the melt viscosity, though, is minor. In contrast, when mixing two powders having highly differing average molecular weights, the resultant finished PVC parts have rough surfaces and flow lines. The cause of this resides in the heterogeneity of the powder. The very low molecular weight PVC granules melt more rapidly in the plasticizing zone of the injection molding machine than the high molecular weight PVC grains, reduce the melt viscosity, and thereby impair (similarly as in the case of adding excessive amounts of lubricant) the plasticizing step for the high molecular weight proportion, leading to flaws in the finished product.
The drawback of powder heterogeneity with regard to molecular weight is avoided in the method of DOS 2,606,934. The latter describes the possibility of varying the molecular weight during VC suspension polymerization by the simultaneous presence of regulating (molecular-weight-lowering) and branching (molecular-weight-raising) materials. The thus-attainable reduction in PVC melt viscosity is related to the quantities of regulating and branching components added. The higher the quantities, the lower the melt viscosity. However, it is impossible, following this procedure, to produce polyvinyl chloride injection-molded parts having a flawless, smooth surface characteristic from powder material having reduced melt viscosity; this has been confirmed by experiments. The cause of this resides in the nature of the branching components, leading to crosslinking of the macro-molecules recognizable by gel particles which are insoluble in cyclohexanone, for example.
DOS 1,943,638 avoids the use of branching components with a process that provides for broadening the molecular weight distribution by polymerization at various temperatures wherein compounds regulating molecular weight can optionally be present. However, the reduction in melt viscosity for products having the same average molecular weight remains small. Improvement in flow is also described in EP 0,137,138 B1 by the addition, based on the conversion, of regulating compounds and by polymerization of at least two different temperatures. In this process, broadening of the molecular weight distribution is likewise achieved. However, the mechanical properties of the finished parts, such as, for example, notched impact strength, leave much to be desired.
An improvement in the impact resistance of PVC can be obtained by the incorporation of rubber or other high polymers viscoelastic at room temperature. This can be done, for example, by mixing on a roll mill at high temperatures or by mixing the dispersions of a polyvinyl chloride and of a visco-elastic polymer or impact-strength-modified PVC. However, even with the use of a low molecular weight polyvinyl chloride, it is still impossible to achieve an adequately low melt viscosity that would permit use of the mixture for the manufacture of large thin-walled molded parts without shear lines.
It has now been discovered surprisingly that by graft copolymerization of VC to preferably 1-20% by weight of a crosslinked polybutyl acrylate in an aqueous dispersion, with the simultaneous use of materials having a chain transfer effect, a product can be obtained having satisfactory grain size distribution and a narrow molecular weight distribution, the low melt viscosity of which makes it suitable for processing, for example, by the injection molding method, and the notched impact strength of which is improved as compared with homopolymers, without, however, having to suffer a loss in heat resistance or surface characteristics of the thus-manufactured molded parts.